This invention relates, in general, to fabrication of semiconductor devices and, more particularly, to fabrication of vertical and lateral isolated semiconductor devices on bonded wafers.
At the present, fabrication of semiconductor devices using buried oxide or in conjunction with wafer bonding is not effective for several reasons, such as complex processing, planarization of a deposited dielectric layer, difficulties of bonding together two semiconductor wafers, and high cost.
As practiced by one method in the prior art, a first semiconductor substrate, made of silicon, is etched to form a plurality of cavities in the semiconductor substrate. These cavities are formed in the semiconductor substrate by any number of processes well known in the art. Generally, a dielectric material such as oxide is then applied to the semiconductor substrate in such a manner that the dielectric material completely fills the plurality of cavities. As a result of the dielectric material completely filling the etched plurality of cavities, surfaces surrounding the plurality of cavities are covered with dielectric material as well. The dielectric material is then planarized; however, planarizing with conventional methods results in several problems, such as particles on the surface of the semiconductor substrate, non-uniformity of the planarized dielectric material, and a high cost of processing the semiconductor substrate through the planarizing processes. The first semiconductor substrate is then bonded to a second semiconductor substrate; however, in bonding the first and second semiconductor substrates together, having the dielectric material cover the surfaces surrounding the plurality of cavities in the first semiconductor substrate, as previously discussed, results in still further problems in the bonding of the two semiconductor substrates. Typically, the problems in bonding the two semiconductor wafers together result in large and small voids between the two semiconductor wafers, which can lead to delamination of the two semiconductor substrates. The delamination of the two semiconductor substrates is a catastrophic defect and removes the two semiconductor substrates from use. Generally, the second semiconductor substrate is then polished back to a suitable thickness, thereby providing buried oxide material under the second semiconductor substrate.
Alternatively, in another method used in the prior art, a dielectric material, such as oxide, is applied or grown on a first silicon substrate. The dielectric material is selectively etched, thereby creating a plurality of openings to the silicon substrate, as well as a plurality of dielectric islands on the silicon substrate. Subsequently, the silicon substrate is processed through a selective epitaxial deposition, wherein silicon is epitaxially and selectively formed in the openings between the islands of oxide on the first semiconductor substrate. However, selective epitaxial deposition is a difficult process to control, and does not form a smooth bondable surface that is planar with the dielectric material. Thus, the epitaxially deposited material must be planarized as previously discussed, thereby resulting in similar problems.
Additionally, it should be noted that formation of the buried dielectric material in both of the methods discussed hereinbefore results in stress being created in the first silicon substrate. This stress is further aggravated by a difference in thermal coefficients of expansion between the dielectric material and a semiconductor substrate when subsequent thermal processing is done.
It can be readily seen that conventional methods for making buried oxide in conjunction with wafer bonding have severe limitations. Also, it is evident that the conventional processes that are used to fabricate buried dielectric materials in conjunction with wafer bonding are not only complex and expensive, but also are not effective processes. Therefore, a method for making a buried dielectric layer that provides vertical and lateral isolation for a semiconductor device on a bonded wafer would be highly desirable.